Conventional reprocessing of spent nuclear fuel from nuclear reactors is a complex and expensive process. There has been substantial interest in developing so called pyro-processing methods that could have lower cost and produce plutonium of relatively low purity, Such low purity plutonium would have applications in molten salt reactors where purity is less critical than in conventionally fuelled reactors.
The processes for pyro-processing separation of uranium and plutonium in spent uranium oxide fuel typically requires conversion of the fuel to metallic form.
The industrially standard way to convert uranium or other actinide oxides to metal is the thermite reaction where the oxide reacts exothermally with calcium metal forming calcium oxide and the actinide metal. This has the drawback of creating large amounts of radioactively contaminated calcium oxide.
A second method which has been studied is the electrochemical reduction of the oxide in an electrolyte by the so called “Cambridge FFC process”. This reduces the solid oxide to solid metal in situ and is commercially used in producing refractory metals. Attempts to use it for spent nuclear fuel have however been frustrated by incomplete reduction of the oxides leading to contamination of subsequent stages in the process with the oxides (Choi and Jeong, Progress in Natural Science: Materials International 25 (2015) 572-582).
A third method was attempted by the US Atomic Energy Commission in the 1960's (Piper, U.S. Pat. No. 3,052,611) where electrochemical reduction of uranium oxide in an electrolyte above the melting point of uranium metal was designed to produce a pure molten uranium metal continuously from an electrochemical reducer—a similar process to that used to smelt aluminium, Unfortunately, this was relatively unsuccessful as the uranium failed to agglomerate, instead producing metal shot coated with uranium oxide. Attempts to overcome this drawback by mixing graphite with the uranium oxide were unsuccessful due to contamination of the uranium with carbon. Further attempts by Mitsubishi in the 1990's led to the need for use of a second lighter molten alloy floating on the molten uranium to prevent contamination of the uranium with oxide (JPH09228089 (A)—1997 Sep. 2002). Since all metals have some solubility in molten uranium this also led to contamination of the uranium metal.
Once conversion of oxide to metal is done, the separation of uranium, other actinides and other metals can be carried out by two processes.
The first is electrochemically, as pioneered at Argonne National Laboratory (OECD/NEA Pyrochemical separations in nuclear applications ISBN 92-64-02071-3). In this process uranium is first electrochemically transferred from an impure metallic anode to an iron cathode where high purity uranium is deposited in a dendritic form. Other actinides remain in the electrolyte and are then transferred by continuing electrolysis into a molten cadmium or bismuth cathode. A major drawback of this process is the dendritic form of the uranium which means it has to be physically transferred from the electrolysis cell with risk of dendrites breaking off and inevitable substantial contamination of the uranium with the electrolyte trapped between and within the dendrites. Extra steps to purify the dendritic uranium are therefore needed.
The second process (eg Kinoshita et al, J. Nuclear Science and Technology, 36:2. 189-197) involves partitioning the impure metal between a low melting point metal such as cadmium or bismuth and a molten salt. Separation is based on the different partitioning behaviour of the different metals in such systems. This approach carries substantial complexity however as recovery of the actinides from the bismuth or cadmium is required and the low solubility of uranium in such metals requires relatively large volumes to be used.
Both of these processes, as so far devised, are complex and involve many stages. None have been commercialised. There remains a need therefore for a simple method of achieving the conversion of spent uranium oxide based reactor fuel into fuel for advanced reactors and especially for molten salt reactors.